The present invention relates to a radiological imaging apparatus and radiological imaging method, and more particularly, to a radiological imaging apparatus and radiological imaging method ideally applicable to positron emission computed tomography (hereinafter referred to as “PET”) and single photon emission computer tomography (hereinafter referred to as “SPECT”).
Radiological imaging is a non-invasive imaging technology to examine physical functions and conformation of a medical examinee. Among typical radiological imaging methods using radiation are X-ray computed tomography, PET and SPECT, etc. X-ray computed tomography irradiates an examinee with radioactive rays radiated from an X-ray source and picks up images of the physical conformation based on the transmittance of radioactive rays in the body of the examinee. Detecting the intensity of X-rays passing through the body using a radiation detector makes it possible to calculate a linear attenuation coefficient between the X-ray source and the radiation detector. From this linear attenuation coefficient, a linear attenuation coefficient of each voxel is calculated using a filtered back projection method described in the IEEE Transactions on Nuclear Science NS volume 21 (issued in 1974, pp.228-229) and this value is converted to a CT value. The radiation source often used for X-ray computed tomography is approximately 80 keV.
PET is a method consisting of administering radiopharmaceutical (hereinafter referred to as “PET radiopharmaceutical”) including matters having a property of concentrating on positron radiateters (15O, 13N, 11C, 18F, etc.) and specific cells in the body to the examinee and examining locations in the body where more PET radiopharmaceutical are consumed. One positron emitted from a positron radiateter in the PET radiopharmaceutical couples with an electron of a neighboring cell to disappear and irradiates a pair of γ-rays (γ-ray pair) having energy of 511 keV. These γ-rays are radiated in directions opposite to each other. Detecting this pair of γ-rays using a radiation detector makes it possible to know between which radiation detectors the positron is emitted. Detecting those many γ-ray pairs makes it possible to identify locations where more PET radiopharmaceutical are consumed. For example, when PET radiopharmaceutical including positron radiateters are created using carbohydrate as a matter having a property of concentrating on a specific cell, these PET radiopharmaceutical concentrate on cancer cells having hyperactive carbohydrate metabolism. This makes it possible to discover cancer focuses. The data obtained is converted to radiation density of each voxel using a method such as the aforementioned Filtered Back Projection. 15O, 13N, 11C and 18F used for the PET are radioisotopes with a short half life of 2 to 110 minutes.
The SPECT administers radiopharmaceutical (hereinafter referred to as “SPECT radiopharmaceutical”) including single photon radiateters to an examinee and detects γ-rays radiated from the radiateters using a radiation detector. The energy of γ-rays radiated from the single photon radiateters often used for inspection using the SPECT is around several 100 keV. In the case of the SPECT, single γ-rays are radiated, and therefore it is not possible to obtain their angle of incidence upon the detector. Thus, angle information is obtained by detecting only γ-rays incident from a specific angle using a collimator. The SPECT administers SPECT radiopharmaceutical including a matter having a property of concentrating on a specific tumor or molecule and single photon radiateters (99Tc, 67Ga, 201Tl, etc.) to the examinee, detects γ-rays generated by the SPECT radiopharmaceutical and identifies locations where more SPECT radiopharmaceutical are consumed. The SPECT also converts data obtained to data of each voxel using a method such as Filtered Back Projection. The SPECT often takes transmission images, too. 99Tc, 67Ga and 201Tl used for the SPECT have a half life longer than that of radioisotopes used for the PET, for example, 6 hours to 3 days.
The aforementioned conventional inspections are carried out independently of one another. Inspections using the PET and SPECT make it possible to know a distribution of the amount of consumption of radiopharmaceutical within an image pickup apparatus. However, because of the absence of information on the correspondence with the physical locations of an examinee, the detailed position of the focus may remain unidentified. Thus, coupling of a PET image or SPECT image with an X-ray computed tomographic image that can identify locations in the body of the examinee is being practiced in recent years. An example of such a radiological imaging apparatus is described in JP-A-7-20245. That is, the radiological imaging apparatus places the image pickup apparatus of the X-ray computed tomographic apparatus and that of the PET apparatus side by side close to each other in parallel to realize quasi-simultaneous imaging. The examinee is laid down on a bed of an examinee holding apparatus and sequentially moved inside both image pickup apparatuses through horizontal movements of the bed. Pictures of the examinee are taken by the image pickup apparatus of the X-ray computed tomographic apparatus and then by the image pickup apparatus of the PET apparatus. In this case, since the time interval between two imaging operations is short and the examinee hardly moves on the bed, it is possible to know a correlation between the PET data and X-ray computed tomographic data, the image data taken by the two image pickup apparatuses. The PET data is coupled with the X-ray computed tomographic data using the information on the correlation and the focus location of the examinee is identified in this way.
JP-A-9-5441 describes a radiological imaging apparatus which also serves as a bed with an image pickup apparatus of an X-ray computed tomographic apparatus placed in parallel just next to an image pickup apparatus of a SPECT apparatus. The X-ray computed tomographic data and the SPECT data which are the image data taken by those image pickup apparatuses are coupled to identify the focus location of the examinee.
The radiological imaging apparatuses described in the above-described publications apparently present a clear positional relationship between two image data pieces, but there is a possibility that the examinee will move between both image pickup apparatuses. Resolution of an image pickup apparatus of a recent PET apparatus is approximately 5 mm and resolution of an image pickup apparatus of an X-ray computed tomographic apparatus is approximately 0.5 mm, an order of magnitude smaller. Because of this, if the examinee moves between both image pickup apparatuses or the angle of the examinee changes, the correlation between image data pieces taken by both image pickup apparatuses becomes unclear. As a result, after reconstructing image data pieces into an image, it is necessary to extract characteristic areas that exist commonly in different images, find a positional relationship between those images from the positional relationship of the characteristic areas and perform positioning on those images. Furthermore, equipped with two image pickup apparatuses each provided with a radiation detector, etc., these radiological imaging apparatuses have a complicated apparatus configuration.